<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style type="text/css">

        /*对div元素，class属性为a的元素，对id为abc的元素都起作用*/
        div, .a, #abc {
            width: 250px;
            height: 35px;
            border: 2px dotted black;
            background-color: #888;
        }
    </style>
</head>
<body>
<div>没有class属性也没有id的div元素</div>
<p class="a">class属性为a的元素</p>
<section id="abc">id为abc元素</section>
</body>
</html>